TheiscSgene is essential for the biosynthesis of 2-selenouridine in tRNA and the selenocysteine-containing formate dehydrogenase H

Abstract

Three NifS-like proteins, IscS, CSD, and CsdB, from Escherichia coli catalyze the removal of sulfur and selenium from L-cysteine and L-selenocysteine, respectively, to form L-alanine. These enzymes are proposed to function as sulfur-delivery proteins for iron-sulfur cluster, thiamin, 4-thiouridine, biotin, and molybdopterin. Recently, it was reported that selenium mobilized from free selenocysteine is incorporated specifically into a selenoprotein and tRNA in vivo, supporting the involvement of the NifS-like proteins in selenium metabolism. We here report evidence that a strain lacking IscS is incapable of synthesizing 5-methylaminomethyl-2-selenouridine and its precursor 5-methylaminomethyl-2-thiouridine (mnm 5 s 2 U) in tRNA, suggesting that the sulfur atom released from L-cysteine by the action of IscS is incorporated into mnm 5 s 2 U. In contrast, neither CSD nor CsdB was essential for production of mnm 5 s 2 U and 5-methylaminomethyl-2-selenouridine. The lack of IscS also caused a significant loss of the selenium-containing polypeptide of formate dehydrogenase H. Together, these results suggest a dual function of IscS in sulfur and selenium metabolism.

Yeast Nfs1p Is Involved in Thio-modification of Both Mitochondrial and Cytoplasmic tRNAs

“…In prokaryotes, IscS-mediated sulfur mobilization is utilized in various processes, including the biosynthesis of ISC, thiamine, and thionucleosides in tRNAs (18,20,41,42). From our results here, we conclude that the yeast IscS homolog, Nfs1p, is also required for the post-transcriptional modification of tRNAs, suggesting that, in addition to the well known essential contribution to ISC formation, eukaryotic Nfs1 also participates in multiple physiologically important reactions as a sulfur donor.…”

The SufE Protein and the SufBCD Complex Enhance SufS Cysteine Desulfurase Activity as Part of a Sulfur Transfer Pathway for Fe-S Cluster Assembly in Escherichia coli

“…2 Based on the crystal structure, the SufE protein has a novel fold consisting of both ␣-helices and ␤-sheets and is a homodimer with the dimer interface comprising residues Glu 21 , Tyr 24 , Trp 20 , Thr 116 , Leu 115 , and Arg 119 , all of which are highly conserved among SufE homologues (Fig. 4A).…”

“…Besides its cysteine desulfurase activity, SufS has also been shown to exhibit a strong selenocysteine lyase activity in vitro (5), although SufS does not appear to be important for selenium metabolism in vivo (24). SufA is a homologue of IscA and can form Fe-S clusters in vitro (25).…”

Substitutions in an Active Site Loop of Escherichia coli IscS Result in Specific Defects in Fe-S Cluster and Thionucleoside Biosynthesis in Vivo

“…All use cysteine and selenocysteine as a substrate in vitro with varying efficiency (14). IscS is required for selenium mobilization for the synthesis of the modified nucleoside mnm 5 se 2 U 1 in tRNA (15), even though SufS utilizes selenocysteine more efficiently in vitro (14). In E. coli, the sufS gene is part of the suf operon that is thought to contribute to Fe-S cluster formation under conditions of cellular stress (16).…”

“…ThiI, in turn, binds tRNA, activates a specific uridine using Mg-ATP, and transfers the sulfur, during which it is oxidized to a disulfide (30). The synthesis of 2-thiouridine requires IscS in vitro (31) and in vivo (15,25,26) and may be similar to s 4 U synthesis with the MnmA protein as the recipient of the IscS persulfide (31). Both mnmA and iscS mutants contain mnm 5 U instead of the thiolated derivative, indicating that the modifications at the 2 and 5 positions are formed independently.…”